The invention relates to temperature measurement radiometry and more particularly to a radiometer combined with a long range reflectometer for accurately measuring the temperature of a remote radiant source.
All bodies at temperatures above absolute zero emit radiation. At low temperatures the emission peaks in the infrared spectral region. For higher temperatures the emission shifts toward shorter wavelength, peaking in the visible spectrum for temperatures approaching that of the sun. Conventional radiometers used for temperature measurements intercept thermal radiation emitted from a radiant source; a calibrated thermopile or photodetector responds to the intercepted radiation, producing an electrical signal which is a measure of the temperature of the radiant source. Radiometric temperature measuring devices are reasonably accurate under idealized or specific conditions. The characteristics of thermal radiation emitted by a radiant source depend, however, not only upon the temperature of the source, but also on the emissivity of the sources' surface. Accurate temperature measurement by radiometric techniques necessitates either knowing or measuring the emissivity, which itself is a function of temperature and wavelength. The present invention accomplishes this necessity by directly measuring the subject surfaces' emissivity and adjusting or compensating the radiometric measurements to determine the objects real temperature. The application of optical radiometers to temperature measurement has also been limited to certain wavelength ranges by interfering media such as the flue gases of a fired furnace. This problem is typically minimized by narrowing the spectral width of detection to a spectral region where the medium interposed between target and radiometer exhibts minimal infrared absorption. This is conventionally achieved by spectral filtering. The shortcoming of such techniques is that the bandwidth of the conventional filter typically exceeds the spectral line spacing of the molecular gases by an order of magnitude with the result that there are usually molecular resonances within any chosen filter band.
The effective optical density of the gas interposed between radiometer and target depends on the number and strength of the molecular resonances within a given filter band width, and the number of molecules per cross-sectional area along the viewing path.
In cases where this effective optical density is too large to be ignored in the radiometer measurement, one can resort to heterodyne detection which yields an ultra narrow radiometer band width. The center frequency of this radiometer band can be tuned to avoid the molecular resonances of the interfering gas altogether.
The art has heretofore recognized several of the advantages of heterodyne mixing a subject signal with a coherent laser signal to increase detection capability within a narrow spectral range. For example, in a technical publication entitled "Heterodyne Detection of a Weak Light Beam", Journal of the Optical Society of America, Volume 56, No. 9, pp. 1200-1206 September 1966, L. Mandel teaches the use of laser heterodyne techniques to detect a weak, spectally narrow light beam from a distant source. The heterodyne principle was also used to detect the 10 .mu.m emission of CO.sub.2 molecules in the atmosphere of the planet Venus; "Heterodyne Detection of CO.sub.2 Emission Lines and Wind Velocities in the Atmosphere of Venus," A. L. Beltz, M. A. Johnson, R. A. McLaren and E. C. Sutton, the Astrophysical Journal 208, pp. 141-L 144 (Sept. 15, 1976).
The present invention advances the combination of a narrow band radiometer with a long range laser reflectometer in applications of remote temperature measurements by optical means. The concurrent measurement of the radiance from a distant hot surface within a narrow spectral band and the determination of the surface emissivity by measurement with the laser reflectometer within that same band is utilized to enhance the accuracy of radiometric temperature measurement.
In one embodiment of the invention an ultra narrow radiometer band widths is attained by the principle of heterodyne detection. In another embodiment a narrow radiometer band width is obtained with an optical filter.
Emissivity measuring devices as exemplified by U.S. Pat. Nos. 4,117,712 and 3,672,221 have demonstrated marginal practical utility and are nonapplicable for large distances between the subject surface and the measurement device and for hot surfaces with substantial radiative self emission. Also, similar to optical pyrometers, these emissivity measuring devices have severely limited application when the subject radiant source is in an optically absorbing medium.